1. Field of the Invention
The invention relates generally to desaturase enzymes that modulate the number and location of double bonds in long chain poly-unsaturated fatty acids (LC-PUFA's). In particular, the invention relates to the alteration of fatty acid profiles using delta 5 desaturase enzymes and nucleic acids encoding such desaturase enzymes.
2. Description of the Related Art
The primary products of fatty acid biosynthesis in most organisms are 16- and 18-carbon compounds. The relative proportion of chain lengths and degree of unsaturation of these fatty acids vary widely among species. Mammals, for example, produce primarily saturated and monounsaturated fatty acids, while most higher plants produce fatty acids with one, two, or three double bonds, the latter two comprising polyunsaturated fatty acids (PUFA's).
Two main families of PUFAs are the omega-3 fatty acids (also represented as “n-3” fatty acids), exemplified by eicosapentaenoic acid (EPA, 20:5, n-3) and the omega-6 fatty acids (also represented as “n-6” fatty acids), exemplified by arachidonic acid (ARA, 20:4, n-6). PUFAs are important components of the plasma membrane of the cell and adipose tissue, where they may be found in such forms as phospholipids and as triglycerides, respectively. PUFAs are necessary for proper development in mammals, particularly in the developing infant brain, and for tissue formation and repair. Arachidonic acid is the principal precursor for the synthesis of eicosanoids, which include leukotrienes, prostaglandins, and thromboxanes, and which also play a significant role in the inflammation process.
Several disorders respond to treatment with fatty acids. Supplementation with PUFAs has been shown to reduce the rate of restenosis after angioplasty. Evidence indicates that PUFAs may be involved in calcium metabolism, suggesting that PUFAs may be useful in the treatment or prevention of osteoporosis and of kidney or urinary tract stones. The majority of evidence for health benefits applies to the long chain omega-3 fats, eicosapentaenoic acid and docosahexaenoic acid (DHA, 22:6, n-3) which are found in fish and fish oil.
PUFAs, such as linoleic acid (LA, 18:2, Δ9, 12) and α-linolenic acid (ALA, 18:3, Δ9, 12, 15), are regarded as essential fatty acids in the diet because mammals lack the ability to synthesize these acids. LA is produced from oleic acid (OA, 18:1, Δ9) by a Δ12-desaturase while ALA is produced from LA by a Δ15-desaturase. However, when ingested, mammals have the ability to metabolize LA and ALA to form the n-6 and n-3 families of long-chain polyunsaturated fatty acids (LC-PUFA). In mammals, the formation of LC-PUFA is rate-limited by the step of Δ6 desaturation, which converts LA to GLA and ALA to SDA. Many physiological and pathological conditions have been shown to depress this metabolic step even further, and consequently, the production of LC-PUFA. To overcome the rate-limiting step and increase tissue levels of EPA, one could consume large amounts of ALA. Alternatively, bypassing the Δ6-desaturation via dietary supplementation with EPA or DHA can effectively alleviate many pathological diseases associated with low levels of PUFA. However, as set forth in more detail below, currently available sources of PUFA are not desirable.
Major long chain PUFAs of importance include DHA and EPA, which are primarily found in different types of fish oil, and ARA, found in filamentous fungi such as Mortierella. For DHA, a number of sources exist for commercial production including a variety of marine organisms, oils obtained from cold water marine fish, and egg yolk fractions. However, there are several disadvantages associated with commercial production of PUFAs from natural sources. Natural sources of PUFAs tend to have highly heterogeneous oil compositions. The oils obtained from these sources therefore can require extensive purification to separate out one or more desired PUFAs or to produce an oil which is enriched in one or more PUFAs.
Other natural limitations favor a novel approach for the production of PUFAs. Weather and disease can cause fluctuation in yields from both fish and other marine sources. Large-scale fermentation of organisms such as Mortierella is expensive. Natural animal tissues contain low amounts of ARA and are difficult to process.